Method for reducing sag in drilling, completion and workover fluids

ABSTRACT

A method is disclosed for reducing sag in drilling and completion fluids and in workover fluids. The method employs a drilling fluid comprising a low molecular weight polyalkyl methacrylate with an average molecular weight ranging from about 40,000 to about 90,000. This additive does not result in significantly increased viscosity of the fluid.

RELATED APPLICATION

[0001] This application is a continuation-in-part of U.S. patentapplication Ser. No. 10/175,256, filed Jun. 19, 2002, pending.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to methods and compositions forreducing the sag of a weight material in a drilling fluid for drillingoperations such as drilling, running casing, and cementing and in fluidsfor workover operations.

[0004] 2. Description of Relevant Art

[0005] A drilling fluid, or “mud” which a drilling fluid is also oftencalled, is a specially designed fluid that is circulated in a wellboreor borehole as the wellbore is being drilled in a subterranean formationto facilitate the drilling operation. As used herein, the term “drillingoperation” shall mean drilling, running casing and/or cementing unlessindicated otherwise. The various functions of a drilling fluid includeremoving drill cuttings from the wellbore, cooling and lubricating thedrill bit, aiding in support of the drill pipe and drill bit, andproviding a hydrostatic head to maintain the integrity of the wellborewalls and prevent well blowouts. Specific drilling fluid systems areselected to optimize a drilling operation in accordance with thecharacteristics of a particular geological formation.

[0006] A drilling fluid typically comprises water and/or oil orsynthetic oil or other synthetic material or synthetic fluid as a basefluid, with solids in suspension. A non-aqueous based drilling fluidtypically contains oil or synthetic fluid as a continuous phase and mayalso contain water dispersed in the continuous phase by emulsificationso that there is no distinct layer of water in the fluid. Such dispersedwater in oil is generally referred to as an invert emulsion orwater-in-oil emulsion.

[0007] A number of additives may be included in such drilling fluids andinvert emulsions to enhance certain properties of the fluid. Suchadditives may include, for example, emulsifiers, weighting agents,fluid-loss control agents, viscosifiers or viscosity control agents, andalkali. Weighting agents are commonly added to increase the density ofthe fluid. Barite or barytes (barium sulphate) are typical weightingagents, although other minerals are also common.

[0008] Viscosifiers are typically added to increase the viscosity of thefluid to facilitate or enhance suspension of weighting agents in thefluid. Organoclays, such as for example bentonites, hectorites and otherswelling clays, chemically treated to enhance their oil wettability, aretypical viscosifiers. Organic polymers and long chain fatty acids mayalso be added to increase viscosity and aid weighting agent suspension.

[0009] Generally, increased viscosity leads to improved suspension ofweighting agents which in turn limits or reduces problematic “sag.”However, excessive viscosity can have adverse effects on equivalentcirculating density (causing it to increase), which can also lead toproblems, particularly in wells where the differences in subterraneanformation pore pressures and fracture gradients are small, as commonlyencountered in deep water wells.

[0010] Under certain well conditions, including without limitation, wellgeometries, temperatures and pressures, a phenomenon called “barite sag”or “sag” can occur. “Sag” is generally a “significant” variation in muddensity (>0.5 lbm/gal) along the mud column, which is the result ofsettling of the weighting agent or weight material and other solids inthe drilling fluid. Sag generally results from the inability of thedrilling fluid under the particular well conditions to provide adequatesuspension properties.

[0011] Suspensions of solids in non-vertical columns are known to settlefaster than suspensions in vertical ones. This effect is driven bygravity and impeded by fluid rheology, particularly non-Newtonian andtime dependent rheology. Manifestation of this effect in a drillingfluid is also known as “sag,” although sag occurs in both vertical andnon-vertical wells.

[0012] Drilling fluids in deviated wellbores can exhibit sag in bothstatic and dynamic situations. In this context, static is a totallyquiescent fluid state, such as when drilling has ceased; dynamic is anysituation where the fluid is exposed to a shear stress, such as forexample during drilling*. As used hereinafter, the term “sag” shall beunderstood to include both static and dynamic sag unless specificallyindicated otherwise.

[0013] Sag can result in formation of a bed of the weighting agents onthe low side of the wellbore, and stuck pipe, among other things. Insome cases, sag can be very problematic to the drilling operation and inextreme cases may cause hole abandonment. As directional drilling anddeviated wellbores become more common if not the norm in the oil and gasindustry, more and improved methods are needed to reduce or eliminatesag, without adding viscosity problems.

SUMMARY OF THE INVENTION

[0014] The present invention provides a method for reducing sag or thesettlement of weighting agents in an oil based or invert emulsiondrilling fluid without significantly increasing the viscosity of thedrilling fluid. The method comprises adding a low molecular weightpolyalkyl methacrylate to the drilling fluid. No copolymer such as vinylpyrrolidone or its equivalent is needed or used. The invention alsoprovides a drilling fluid comprising the additive. As used herein, theterm “drilling fluid” shall be understood to include fluids used indrilling, cementing, casing, workover and/or other similar downholeoperations.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0015] In the present invention, an oil or invert emulsion baseddrilling fluid comprising a low molecular weight polyalkyl methacrylateadditive is used in drilling operations or workover operations. Thefluid provides good suspension of weighting agents without excessiveviscosity and shows a reduced rate of sag when compared to prior artfluids.

[0016] The effective average molecular weight range of the polyalkylmethacrylate is about 40,000 to about 90,000. An example polyalkylmethacrylate within this weight range is commercially available underthe tradename VISCOPLEX® 1-304, available from ROHMAX or Rohm GmbH ofGermany. Preferred concentrations of polyalkyl methacrylate in thedrilling fluid are in the range of about 0.1 ppb to about 10 ppb. Atthese concentrations, this additive of the present invention does notsignificantly increase the viscosity of the drilling fluid when measuredat either about 120 degrees Fahrenheit and ambient pressure or attemperatures up to about 400 degrees Fahrenheit and 20,000 psigpressure. If desired, one or more viscosifiers may optionally be addedto slightly increase the viscosity of the drilling fluid. Alternatively,a compound or material comprising polyalkyl methacrylate in aconcentration ranging from about 0.1 ppb to about 10 ppb and aviscosifier in a concentration that will slightly increase the viscosityof the drilling fluid, may be added to drilling fluid.

[0017] Sag factors achieved using this low molecular weight polyalkylmethacrylate additive range from about 0.50 to about 0.52. Typical priorart drilling fluids presently in use in the oil and gas industry exhibitsag factors ranging from about 0.51 to about 0.55, where a sag factor of0.50 indicates no settlement of weighting agent (i.e., no sag).Copolymers with the polyalkyl methacrylate are not needed in the presentinvention.

[0018] U.S. Pat. No. 6,204,224, issued Mar. 20, 2001 to Quintero et al.previously reported use of a copolymer of a high molecular weightpolyalkyl methacrylate and vinyl pyrrolidone for Theologicalmodification and filtration control for oil or invert emulsion based,and particularly ester and synthetic based, drilling fluids. Thesecopolymers are said to be capable of building a viscoelastic Theologicalstructure which serves to prevent or reduce barite sag and providecuttings transport. The patent includes data comparing the copolymer ofhigh molecular weight polyalkyl methacrylate and vinyl pyrrolidone witha copolymer of low molecular weight polyallyl methacrylate and vinylpyrrolidone which indicates that the latter yielded poor results incontrast to the claimed high molecular weight polyalkyl methacrylate andvinyl pyrrolidone copolymer. The data in the patent followed traditionalexpectations that the more viscous fluid would provide the bettersuspension properties and be less likely to sag. Addition of the highmolecular weight polyalkyl methacrylate and vinyl pyrrolidone copolymerto the drilling fluid significantly increased the viscosity of thefluid, which resulted in the drilling fluid being a highly viscousfluid.

[0019] Notwithstanding the teaching of U.S. Pat. No. 6,204,224 toQuintero, et al., the present invention departs from traditionalthinking and unexpectedly provides a formulation that yields superiorresults in reducing sag without added viscosity and thus withoutconsequent problems that highly viscous fluids can cause. The advantagesof the present invention are demonstrated by the test data below.

EXPERIMENTS

[0020] Initially, samples of field drilling mud were either static agedor hot rolled at 400 degrees Fahrenheit to evaluate the mud at 50degrees Fahrenheit higher than the mud was originally engineered towithstand. The mud generally held up well, although the differencebetween the top and bottom density results were excessive. To improvebarite (weighting agent) suspension, two different products were addedto samples (one product per sample) for comparison in concentrations of1.0 lb/bb. These two products were hydrogenated castor oil andVISCOPLEX® 1-304 additive. Data from this experiment is shown in Table 1below. TABLE 1 Field Mud Sample bbl 1.0 1.0 1.0 Hydrogenated Castor Oillb — 1.0 — VISCOPLEX ® 1-304 Additive lb — — — 1.0 Static Aged @ 400 °F. hour 16 16 16 Plastic Viscosity cP 58 75 80 74 Yield Point lb/100 ft²22 29 23 27 10 second Gel lb/100 ft² 14 24 20 24 10 minute Gel lb/100ft² 19 40 38 37 Fann 35 dial readings @ 120° F. Θ600 138 179 183 175Θ300 80 104 103 101 Θ200 60 77 76 74 Θ100 38 48 46 48 Θ6 13 16 14 16 Θ310 14 12 14 Weight lb/gal 18.0 18.0 18.0 Top Oil Separation % 3.0 3.02.5 Gelled/Settling y/n y/n y/n y/n Top Stratification sg 1.87 2.05 2.10Middle Stratification sg 2.10 2.21 2.17 Bottom Stratification sg 2.292.32 2.22

[0021] The concentration of each additive—the hydrogenated castor oiland the VISCOPLEX® 1-304 additive—was increased in additional samples offield mud to 2.0 lb/bbl. As the data in Table 2 indicates, this additionhad the desired effect of reducing settlement (e.g., sag) during staticaging without adverse effects on (i.e., without increasing) eitherviscosity or fluid loss. Of these two additives, the better results wereseen with VISCOPLEX® 1-304 additive and so the studies were conductedonly with VISCOPLEX® 1-304 additive. TABLE 2 Field Mud Sample bbl 1.01.0 1.0 Hydrogenated Castor Wax lb — 2.0 — VISCOPLEX ® 1-304 Additive lb— — 2.0 Static Aged @ 400 ° F. hour 16 16 16 Plastic Viscosity cP 54 6075 62 Yield Point lb/100 ft² 22 27 35 28 10 second Gel lb/100 ft² 13 1721 20 10 minute Gel lb/100 ft² 18 42 49 44 Electrical Stability volt —654 776 878 HTHP Fluid Loss @ 400° F. ml/30 min — — 2.2 2.0 Fann 35 dialreadings @ 120° F. Θ600 130 147 185 152 Θ300 76 87 110 90 Θ200 57 65 8369 Θ100 36 42 53 46 Θ6 12 14 17 17 Θ3 10 12 15 14 Weight lb/gal 18.018.0 18.0 Top Oil Separation % 2.5 2.5 1.5 Gelled/Settling y/n y/n y/ny/n Top Stratification sg 1.94 2.01 2.12 Middle Stratification sg 2.122.16 2.19 Bottom Stratification sg 2.29 2.29 2.24

[0022] Laboratory samples of a drilling mud approximately like the fieldmud were prepared. Different concentrations of VISCOPLEX® 1-304 additivewere added to the samples—1.0 lb/bbl concentration in one sample and 2.0lb/bbl concentration in another sample. These samples were static agedat 400 degrees Fahrenheit for 16 hours. The treated laboratory samplesshowed low density variations and minimal top oil separation, with the2.0 lb/bbl concentration giving slightly better results. The data fromthis experiment is shown in Table 3 below. The products in the tablesdesignated by tradenames EZ MUL® NT, INVERMUL® NT, DURATONE®, andSUSPENTONE™ are available from Halliburton Energy Services, Inc. inHouston, Tex., U.S.A. The product designated as BENTONE® 38 is availablefrom Rheox. These products are common additives for completing adrilling fluid and comprise emulsifiers, Theological control additives,suspension agents and the like. TABLE 3 MUD SAMPLE COMPOSITION Total DF1 (invert emulsion base) bbl 0.393 0.393 0.393 EZ MUL ® NT ¹ ppb 15.015.0 15.0 INVERMUL ® NT ² ppb 3.0 3.0 3.0 DURATONE ® HT ³ ppb 20.0 20.020.0 Lime ppb 10.0 10.0 10.0 BENTONE ® 38 ppb 2.0 2.0 2.0 Water bbl0.102 0.102 0.102 CaCl₂ ppb 19.01 19.01 19.01 SUSPENTONE ™ ⁴ ppb 2.0 2.02.0 Barite ppb 559.93 559.9 559.93 RM ™ 63 ⁵ ppb 1.0 1.0 1.0 VISCOPLEX ®1-304 Additive ppb — 1.0 2.0 Hot rolled @ 400° F. hour 16 Static Aged @400° F. hour 16 16 16 MUD SAMPLE PROPERTIES Plastic Viscosity cP 65 6260 62 61 Yield Point lb/100 ft² 25 19 23 16 17 10 second Gel lb/100 ft²15 12 15 10 14 10 minute Gel lb/100 ft² 26 31 20 30 32 Fann 35 dialreadings @ 120° F. Θ600 155 143 143 140 139 Θ300 90 81 83 78 78 Θ200 6859 62 57 57 Θ100 44 36 38 34 35 Θ6 15 10 12 8 9 Θ3 13 9 10 7 7 Weightlb/gal 18.0 18.0 18.0 Top Oil Separation % 3.5 2.5 2.0 Gelled/Settlingy/n y/n y/n y/n Top Stratification sg 2.11 2.16 2.15 MiddleStratification sg 2.22 2.18 2.17 Bottom Stratification sg 2.36 2.22 2.20

[0023] One sample of the laboratory mud without the VISCOPLEX® 1-304additive and one sample of the laboratory mud with 2.0 lb/bbl VISCOPLEX®1-304 additive were placed in hot roll cells and left static in an ovenfor 96 hours at 400 degrees Fahrenheit. Though there was top oilseparation and settlement in both cells, the sample with the VISCOPLEX®1-304 additive showed significantly better results, as can be seen inTable 4. TABLE 4 MUD SAMPLE COMPOSITION: Total DF 1 (invert emulsionbase) bbl 0.393 0.393 EZMUL ® NT ppb 15.0 15.0 INVERMUL ®NT ppb 3.0 3.0DURATONE ®HT ppb 20.0 20.0 Lime ppb 10.0 10.0 BENTONE ®38 ppb 2.0 2.0Water bbl 0.102 0.102 CaCl₂ ppb 19.01 19.01 SUSPENTONE ™ ppb 2.0 2.0Barite ppb 559.93 559.93 RM 63 ™ ppb 1.0 1.0 VISCOPLEX ™ 1-304 Additiveppb — 2.0 Static Aged @ 400° F. hour 96 96 MUD SAMPLE PROPERTIES PlasticViscosity cP 60 63 65 Yield Point lb/100 ft² 23 13 15 10 second Gellb/100 ft² 15 13 15 10 minute Gel lb/100 ft² 20 32 35 ElectricalStability volt — 799 833 Fann 35 dial readings @ 120° F. Θ600 143 139145 Θ300 83 76 80 Θ200 62 54 57 Θ100 38 30 34 Θ6 12 7 8 Θ3 10 5 7 Weightlb/gal 18.0 18.0 Top Oil Separation % 20.0 15.0 Gelled/Settling y/n y/ny/n Top Stratification sg 1.84 2.0 Middle Stratification sg 2.36 2.20Bottom Stratification sg 2.70 2.50

[0024] A sample of the field mud sample without VISCOPLEX® 1-304additive and a sample of the field mud sample with 2.0 lb/bbl VISCOPLEX®1-304 additive were sheared on a Silverson mixer over a 60 minute periodbefore being static aged at 400 degrees Fahrenheit for 16 hours. Theresults, shown in Table 5, indicated no significant difference betweenthe two samples, which indicated that VISCOPLEX® 1-304 additive canwithstand shear. TABLE 5 Field Mud Sample Sheared for 1 hour bbl 1.0 —VISCOPLEX ® 1-304 Additive ppb — 2.0 Static Aged @ 400° F. hour 16 16Plastic Viscosity cP 70 70 Yield Point lb/100 ft² 27 28 10 second Gellb/100 ft² 22 26 10 minute Gel lb/100 ft² 39 41 Electrical Stabilityvolt 838 1013 Fann 35 dial readings @ 120° F. Θ600 167 168 Θ300 97 98Θ200 72 73 Θ100 44 47 Θ6 14 17 Θ3 12 15 Weight lb/gal 18.0 18.0 Top OilSeparation % 3.5 2.5 Gelled/Settling y/n y/n y/n Top Stratification sg2.10 2.12 Middle Stratification sg 2.18 2.14 Bottom Stratification sg2.22 2.17

[0025] Fann 70 rheological profiles were carried out on samples of bothfield mud without VISCOPLEX® 1-304 additive and field mud with 2.0lb/bbl VISCOPLEX® 1-304 additive. After the samples reached the maximumtemperatures and pressures desired, the samples were left for 4.5 hoursat these maximum temperatures and pressures before instrument readingswere re-taken

[0026] In addition to the Fann 70, the viscosity of both samples wasmeasured on a Fann 35 at minus 5 degrees Centigrade to simulate coldclimates. These tests showed no detrimental effects the addition ofVISCOPLEX® 1-304 additive. The test data is reported in Tables 6 and 7below. TABLE 6 FANN 70 RHEOLOGICAL PROFILE Field Mud Sample Temperature(° C.): 50 100 150 204 204  −5 Pressure (psi): 0 5000 11000 17500 17500Fann 35 4.5 Hours 600 rpm 132 107 102 118 103 O/S 300 rpm 75 64 61 72 53O/S 200 rpm 57 50 48 55 37 249 100 rpm 38 36 34 37 21 148  60 rpm 28 2827 28 16 —  30 rpm 22 22 22 22 10 —  6 rpm 13 15 15 13 5  35  3 rpm 1113 13 10 3  29 Plastic Viscosity, cp 57 43 41 46 50 — Yield Point,lb/100 ft² 18 21 20 26 3 — Gel, 10 sec. lb/100 ft 12 13 13 10 3  37 Gel,10 min. lb/100 ft 15 16 15 13 6  61

[0027] TABLE 7 FANN 70 RHEOLOGICAL PROFILE Field Mud Sample + 2 lb/bblVISCOPLEX ® 1-304 Additive Temperature (° C.): 50 100 150 204 204  −5Pressure (psi): 0 5000 11000 17500 17500 Fann 35 4.5 Hours 600 rpm 144123 96 99 97 O/S 300 rpm 84 79 63 60 52 O/S 200 rpm 65 64 52 47 39 253100 rpm 44 47 39 34 23 152  60 rpm 34 39 32 25 17 —  30 rpm 25 31 26 2111 —  6 rpm 16 22 19 14 6  36  3 rpm 14 21 17 11 3  30 PlasticViscosity, cp 60 44 33 29 45 — Yield Point, lb/100 ft² 24 35 30 31 7 —Gel, 10 sec. lb/100 ft 14 21 17 11 3  37 Gel, 10 min. lb/100 ft 16 23 1914 6  63

[0028] Several apparatus and procedures are available for measuring sag.A typical example is stratification testing to determine or evaluate the“sag” performance of a drilling fluid, i.e., the fluid's tendency to“sag.” The procedure used in stratification testing is as follows:

[0029] A sample of the fluid or formulation is sealed within a stainlesssteel cell and placed vertically in an oven maintained at the expectedmaximum temperature of the well. The cell is usually pressurized usingnitrogen to prevent loss of volatiles from the fluid. After the testperiod, typically 16 hours, the cell is depressurized and the volume of“top” (i.e., separated) oil is measured and expressed as a percentage ofthe total fluid volume. After removal of the top oil, the density of thefluid is measured at three depths—top, middle, and bottom. A “sagfactor” is calculated by dividing the bottom density by the sum of thetop and bottom densities. A sag factor of 0.5 indicates no settlement ofweighting agents.

[0030] Stratification data is reported for sag calculations in Tables1-5 above. As previously noted, the sag factors for the fluid testedwith VISCOPLEX® 1-304 additive ranged from about 0.50 to about 0.51,generally indicating that sag either did not occur or did not occur inan appreciable or significant amount.

[0031] The viscosity of drilling fluid is typically measured by use ofconcentric cylinder viscometers such as the Fann 35SA, available fromFann Instruments in Houston, Tex., U.S.A. The viscosity of fluids underdownhole conditions, i.e., temperatures up to about 500 degreesFahrenheit and pressures to 20,000 psig, is typically measured using aninstrument such as the Fann 70, available from Fann Instruments inHouston, Tex., U.S.A. Viscosity data is reported in all of the tablesabove and comparison of the viscosities of the fluids with and withoutVISCOPLEX® 1-304 additive indicates that the addition of VISCOPLEX®1-304 additive to the fluid did not appreciably or significantly alterthe viscosity of the fluid.

[0032] These experiments indicate that a low molecular weight polyalkylmethacrylate may be added to oil or invert emulsion based drillingfluids to obtain improved suspension properties or reduced sag withoutaltering the viscosity of the fluid. Further, this additive does notseem to significantly alter or increase the fluid loss experienced withuse of the fluid and the additive provides a further benefit ofimproving the static age results seen with the fluid.

[0033] The foregoing description of the invention is intended to be adescription of preferred embodiments. Various changes in the details ofthe described drilling fluid and method can be made without departingfrom the intended scope of this invention as defined by the appendedclaims.

We claim:
 1. A method for reducing sag in an oil or invert emulsionbased drilling fluid, said method comprising adding to said fluid a lowmolecular weight polyalkyl methacrylate.
 2. The method of claim 1wherein the polyalkyl methacrylate has an average molecular weightranging from about 40,000 to about 90,000.
 3. The method of claim 1wherein said fluid has a sag factor in the range of about 0.50 to about0.52.
 4. The method of claim 3 wherein said sag factor is determinedaccording to stratification testing.
 5. The method of claim 1 whereinthe viscosity of said fluid before and after said addition of polyalkylmethacrylate is substantially the same.
 6. The method of claim 1 whereinsaid polyalkyl methacrylate is provided in said fluid in a concentrationranging from about 0.1 ppb to about 10 ppb.
 7. The method of claim 1wherein said drilling fluid is used in drilling or completionoperations.
 8. The method of claim 1 wherein said drilling fluid is usedin workover operations.
 9. A drilling fluid comprising an oil or invertemulsion base, weighting agents and a low molecular weight polyalkylmethacrylate.
 10. The drilling fluid of claim 9 wherein said polyalkylmethacrylate has an average molecular weight ranging from about 40,000to about 90,000.
 11. The drilling fluid of claim 9 wherein saidpolyalkyl methacrylate has a sag factor in the range of about 0.50 toabout 0.52.
 12. The drilling fluid of claim 9 wherein polyalkylmethacrylate is provided in said fluid in a concentration ranging fromabout 0.1 ppb to about 10 ppb.
 13. The drilling fluid of claim 9 furthercomprising emulsifiers.
 14. The drilling fluid of claim 13 furthercomprising at least one additive selected from the group comprisingrheological additives, fluid loss control additives, suspension agents,and viscosifiers that increase the viscosity of the drilling fluidslightly.
 15. A method for improving the suspension properties of an oilor invert emulsion based drilling fluid without significantly increasingthe viscosity of said fluid, said method comprising adding to said fluidan additive comprising a low molecular weight polyalkyl methacrylate.16. The method of claim 15 wherein said polyalkyl methacrylate isprovided in a concentration ranging from about 0.1 ppb to about 10 ppb.17. The method of claim 15 wherein said polyalkyl methacrylate has anaverage molecular weight ranging from about 40,000 to about 90,000. 18.The method of claim 15 wherein said fluid is used in drillingoperations, completion operations, or workover operations in a boreholepenetrating a subterranean formation.
 19. The method of claim 15 whereinsaid fluid further comprises a viscosifer that increases the viscosityof the fluid slightly.